Basic Electrical Circuits: Instrumentation

Hi everyone, time for a new post again and this time round we will have a quick look at instruments that will give you an analogue signal.

Just before we get into that, a reminder to have a look at the Manual for Electricians Volume 1. This is a compact softcover book that you will be able to use as a reference. You will find this at

Right, let’s get cracking with this week’s post.

When we work with the analogue value, you will find some instruments work on a 4-20mA signal, others 0-10V and also you could have 0-20mA. Make sure you use the right signal to match the analogue card of the PLC you may use.

So what does that mean? Once again, we will keep this in plain English and make it easy to follow. Let’s say you are measuring the level of a tank. The tank will hold say 10000L of water when full. The device output is 4-20mA and you now have to set it up to return the correct value.

What you need to do is to set the Engineering Value in the PLC code. This value will be 0 to 10000. We determine this by the capacity of the tank. This now means that when you receive a signal of 4mA from the instrument, the tank will be at 0 and when the signal is 20mA, the tank is at 10000. Keep in mind when performing your testing that 12mA will read 50% of the engineering value (In this case 5000L)

Once you have this set up and getting the required readings, you can set up your control circuit to perform certain functions at various levels like starting pumps, opening valves etc. Something to remember when setting up your low and high level alarms, give yourself a buffer. When you set up your low level, set two levels namely Low and Low Low. In other words the Low alarm will notify you when the levels gets below the setting and you still have sufficient time to react. When you get to the Low Low alarm, this could be the critical point and will shut associated equipment down. Do the same when you set up the high level alarms.

Hope this is of use to you and until next week, remember that there is no substitute for safety! Think before you do something that could result in an injury. Find methods to perform the task in a safe manner!

Till then, have a great week